Alkyl polyglycosides have been known for at least sixty years. They are nonionic surfactants with low toxicity and gentleness to the skin. They can be made from renewable resources and are rapidly degraded in the environment.
Early processes for the preparation of alkyl polyglycosides were two-step processes. The first step comprised the reaction of a lower alcohol having 1 to 6 carbon atoms with a source of saccharide in the presence of an acid catalyst to form the alkyl polyglycoside. These alkyl polyglycosides do not have useful surfactant properties. Since water is miscible with, or highly soluble in, the alcohols, the reaction mixture can contain substantial quantities of water (see U.S. Pat. No. 4,721,780). The higher alkyl polyglycosides in which the aliphatic group contains from 7 to 22 carbon atoms, were prepared by transacetalization of the lower glycosides with higher fatty alcohols under substantially anhydrous conditions.
More recently, alkyl polyglycosides having alkyl groups with from 7 to about 22 carbon atoms have been prepared by a "direct process". In the "direct process" a long chain fatty alcohol is reacted with a source of saccharide in the presence of an acid catalyst under conditions in which the water formed in the reaction is removed as quickly as it is formed to maintain the water content of the reaction mixture at as low a level as is reasonably possible. The water formed in the reaction is only slightly soluble in the fatty alcohol and any undissolved water results in the rapid formation of unwanted byproducts. The parameters of the "direct process" were set out in U.S. Pat. No. 3,839,318, which is incorporated herein by reference. Other patents such as U.S. Pat. Nos. 4,939,245, 4,950,743 and 5,003,057 also describe the "direct process" and are incorporated herin by reference.
In the direct process an anhydrous source of saccharide or a hydrous saccharide source mixed with an alcohol, present in stoichiometric excess, is heated under reduced pressure to remove the water. An acid catalyst is added to the mixture of saccharide source and fatty alcohol and after the water has been substantially eliminated, the mixture is heated under reduced pressure to form the alkyl polyglycoside. As an alternative, a slurry of anhydrous saccharide and alcohol can be formed. The slurry is then added to a heated second portion of alcohol. It is preferred to use a hydrous saccharide source since hydrous saccharide sources are much less expensive than the anhydrous material.
Sources such as 70% high dextrose syrup being liquids, are easier to handle, easier to pump and significantly less expensive then the anhydrous sources presently being used, but have not been used because of the inherent problems caused by the water present. Water in the reaction is only slightly soluble in the fatty alcohol and any undissolved water results in the rapid formation of unwanted byproducts. Also high dextrose syrup is less pure then other forms of dextrose presently being used. It is therefore desirable to find a method of producing alkyl polyglycoside which can use a hydrated saccharide, such as high dextrose syrup, as a source of dextrose as a raw material in the process.